1. Field of the Present Disclosure
The present disclosure is directed to closed-head bone screws for securing spinal instrumentation. Both monaxial and polyaxial screws are described.
2. Related Art
Spinal instrumentation may be used to treat a variety of conditions, including injury and deformity due to trauma or congenital defect. Spinal instrumentation may also be used in a spinal fusion procedure to alleviate the difficulties caused by a damaged or herniated intervertebral disc. Spinal instrumentation typically includes a number of bone screws for attaching to individual vertebrae. The pedicle is a preferred attachment point because it is easily accessible from the back of the patient and it offers a great depth of cortical bone. Once the pedicle screws are in place, one or more rods are placed vertically, substantially parallel to the long axis of the spine, and connected to the pedicle screws. Each rod is usually connected to at least two screws, each of which is attached to a different vertebra. The entire construct serves to properly align and/or separate the vertebrae. It may be supplemented with bone grafts, bone cement, or the like, to promote healing, long-term stability, and the like.
Traditionally, spinal instrumentation was installed via open-back surgery. This type of procedure tended to cause extensive trauma to the patient, resulting in long and painful recovery times. In recent years, a shift has been made toward minimally invasive surgery techniques. In minimally invasive surgery, the surgeon makes only one to a few small incisions and uses special tools to insert devices, observe his progress, and perform other activities in the surgical site. Minimally invasive surgery techniques frequently result in much less injury to the patient and improved healing and recovery times.
The pedicle screws commonly used in instrumentation procedures utilize an open-head design to connect to the rods. This head has a flat base with an arm rising on either side, giving it a U-shaped profile. In a monaxial design, the bone screw itself may be inserted through an opening in the base and then inserted into the bone. The rod is placed between the upright portions of the open head, and a locking clamp, which connects to the upright portions, is placed over the rod to lock it in place. Polyaxial designs are similar, but the connection between the head and the screw allows the head to twist and rotate relative to the bone screw.
Open-head polyaxial and monaxial screws suffer from certain shortcomings. The size of the head scales with the size of the bone screw. In situations that call for a larger screw, the surgeon may need to make more or larger incisions to accommodate the larger profile of the open-head screw. Conversely, if the surgeon will not or cannot enlarge the incision, he may instead use a smaller screw that may not have the strength necessary for the particular application at hand. In addition, smaller polyaxial screws, with their smaller heads, may be able to achieve greater relative angles between the head and the screw than larger screws. This is another reason a surgeon might select a smaller screw that may not be perfectly suited for the particular task at hand. Furthermore, having to maintain stocks of screws and heads in a variety of sizes is cumbersome, costly, and error-prone.
Accordingly, there is a need for low-profile, high-strength polyaxial and monaxial bone screws.